WASHINGTON -- The National Federation of Independent Business, one of the lead plaintiffs in a Supreme Court lawsuit against the Affordable Care Act, refuses to disclose the names of its mega-donors, prompting critics to question the organization's claim to represent small businesses.

The NFIB touts itself as "the voice for small business." But in 2010 and 2011, it received 10 donations totaling more than $10 million from anonymous donors, according to a recent analysis of their tax records by Public Campaign and Alliance for a Just Society.

NFIB has bankrolled the high-profile lawsuit, National Federation of Independent Business v. Sebelius, which the Supreme Court is set to decide with two other health reform cases on Thursday. In turn, it appears, the organization received large donations from undisclosed sources. And while the word "independent" is in the organization's name, NFIB's multimillion-dollar independent expenditures and campaign donations have benefited almost exclusively Republicans.

When asked to disclose its donors on Wednesday, NFIB spokeswoman Jean Card told The Huffington Post in an email: "We are not required to do so by law."

"This shows unambiguously that NFIB is a sham organization, and it's a scam that they ask actual small businesses to pay dues when the group is owned by millionaires, billionaires and big corporations," said Ethan Rome, executive director of Health Care for America Now. "The small businesses of America, the ones who join NFIB in good faith, are being taken for a ride by the big-moneyed interests who want to use NFIB as a front group."

In April, the Washington Postrevealed that NFIB received $3.7 million from Crossroads GPS, the conservative 501(c)(4) nonprofit group affiliated with GOP consultant Karl Rove.

"Crossroads GPS contributions were not used to fund the healthcare lawsuit," NFIB spokeswoman Card said in her email. "NFIB has always been, and will always remain, true and accountable to its membership of America’s smallest businesses."

Rep. Raúl M. Grijalva (D-Ariz.), co-chair of the Congressional Progressive Caucus, said on the House floor Wednesday morning that NFIB is a "front group."

"The president of the group says he’s doing this to help small businesses," Grijalva said. "We asked him who gave him several recent million-dollar-plus donations that have helped fund his lawsuit. He refused to answer. ... NFIB won’t tell the truth about who it represents or how big it is. What does it have to hide?"